Methods for quantifying magnesium ion in a sample includes a method using an ion-selective electrode, and atomic absorption method. Ion-selective electrode is a sensor which selectively responds to the target ion and converts the response to electrochemical information, and the ion activity is determined from the response potential. Atomic absorption method is a method in which vaporized metal atoms are irradiated with a light having a specific wavelength and the metal atoms are quantified from the amount of the absorption. Both of these methods have good sensitivity and accuracy. However, it is pointed out that these methods require pretreatment, and real time monitoring cannot be carried out.
On the other hand, similar to the above-mentioned atomic absorption method, methods for quantifying the target substance in a sample utilizing the process of absorption of light or emission of light by the substance are now rapidly developing with the development of spectroscopy. Among these, fluorometry is now widely used as one of the methods enabling trace of kinetic behavior of metal ions in the body, because the pretreatment is simple, real-time measurement can be attained, and because of the rapid progress of the measuring apparatuses such as fluorescence microscope and confocal laser microscope. Particularly, fluorescent probes for measuring calcium ion have the largest share in this field, and a number of calcium ion-selective fluorescent probes have been synthesized. By virtue of these probes, the kinetic behaviors of magnesium ions in the cells of nerve, muscle and organs are imaged, and so the probes greatly contribute to the fields of medicine and biology.
Magnesium ion is an important metal ion in the body, which acts as a buffering agent, and which controls enzyme reactions and photosynthesis reactions in green plants. In case of the above-mentioned calcium ion, a wide variety of fluorescent pigment molecules have been developed and the behaviors of calcium ions in the body have been clarified. In contrast, no effective fluorescent probe for magnesium ion has been developed. The reasons therefor include that the hydration energy of magnesium ion is greater than that of calcium ion so that a large binding constant cannot be obtained in water, and that competition reaction by calcium ions occurs, so that a complex cannot be formed selectively with magnesium ion.